The present invention relates to drying and cooling functions all in a single unit.
A great number of industrial drying processes invariably require a subsequent unit operation step, cooling. Cooling of a product from an industrial dryer may be required for a variety of reasons; material handling limitations, product stability considerations or possible additional process steps that may require a cooled product.
For applications wherein a rotary dryer is the equipment of choice, cooling generally involves a stand alone rotary cooler following the rotary dryer in series as separate units.
U.S. Pat. No. 4,071,962 details a combination rotary dryer-cooler wherein the drying and cooling occurs within a single rotary vessel. The weakness to this design relative to the present invention lies in the fact that the drying and cooling gases are combined at some midpoint within the rotary vessel. For certain applications, the cooling of dryer gases may result in water vapor condensation. Furthermore, control of the process may prove difficult since the two processes may oppose each other. The higher the drying demand, the less availability of cooling air since this cooling air is induced by a common source. Additionally, the higher the flow of drying gases, the hotter the product while the lower the capacity of cooling gases induced.
U.S. Pat. No. 5,857,516 discloses an indirect solids to fluid (typically water) heat exchanger. This design discloses a solids flow across heat exchange surfaces by gravity. The inherent advantage to the present invention lies in the much higher heat transfer coefficients that can be expected by aerated solids contacting heat exchange surfaces. Relative to quasinstatic sliding flow, the introduction of aeration air leads to potential evaporative cooling and always, direct convection.
The present invention comprises an integrated device that combines the drying and cooling functions in a single vessel. The present invention takes full advantage of a component common to most rotary dryers, the discharge hood or breaching. The discharge breaching is a stationary housing that joins the rotating vessel to stationary product handling equipment and exhaust ductwork (in the case of parallel flow designs) or heat source (in the case of counter-current designs). The discharge breaching may also serve a secondary purpose of being a large enclosure wherein gas velocities are reduced thereby acting as a dropout box for solids entrained in the gas stream. In the present invention, this component can be readily adapted to perform a tertiary function, product cooling.
The design of the present invention is based on modeling an airswept, direct convection rotary cooler wherein solids are in intimate contact with ambient or conditioned air. Since the breaching is stationary, a motive force for conveying the solids onward was sought. The dilemma was overcome since discharge breechings incorporate sloped sides for advance of material downward by gravity. The basic premise of the design was to use a gentler slope on the material side of the breaching face than otherwise required and then assist conveyance by the cooling air-performing aeration.
The mass of air required for cooling is based on the thermal load and the air being distributed evenly to the solids through a perforated plate or screen that becomes the slide surface of the discharge breaching. The cross sectional area of this aerated surface is determined by fluid flow characteristics such as pressure drop considerations and allowable upward gas velocity with respect to entertainment of solids. Additional considerations such as residence time for the solids in this cooling zone must also be taken into account when performing a design on this plate. The orientation and angle of this plate will be a function of rheology for any specific material.
There are many secondary benefits to the design of the present invention. Since the breaching is stationary, indirect cooling tubes could be incorporated without the need for additional structural support or rotary joints as would be encountered incomparable rotary water tube coolers. Since the solids are aerated and dynamic, solids to tube wall heat transfer coefficients are greatly enhanced from the more static, rotary water tube cooler design. Beat transfer can occur in a number of modes.
Direct convection between solids and air flowing perpendicular to slide face (aeration)
Direct convection for solids falling from rotary dryer through the up-running air flow
Conduction between solids and slide surface
Conduction between solids and optional indirect cooling tube surfaces
Conduction between up-running cooling air and optional tube walls
Evaporative cooling between hot moist solids and cool dry air
Another benefit to utilizing the stationary design of the discharge breaching lies in the fact that separation between drying and cooling gases can readily be accomplished by incorporating baffles. The drying and cooling gases can be combined downstream in a common gas cleaning system if advantageous, or kept separate if so desired.
Critical to this design is the ability of the rotary dryer to discharge its product uniformly across the cooling grid surface. It is the intent of the present invention to incorporate two proven concepts, a serrated scroll ring or a trommel screen. In another embodiment, a hybrid combination of either or both in conjunction can be used.
The design of the present invention also lends itself well to augment a drying or heating function as opposed to product cooling. The design could also be adapted as a secondary dryer for slightly undersized rotary dryers in either new installations or as a xe2x80x9cbolt onxe2x80x9d retrofit for existing dryers.
The present invention provides improved heat transfer. The present invention provides lower gas volumes than pure convention designs. The present invention provides improved process-control through an independent supply fan and water flow. The present invention uses indirect heat exchange and is highly dynamic like an aerated fluid bed. There is a separation of drying and cooling gases. Further, the present invention can work by gravity flow. The present invention introduces cooling air to a component that is present on any dryer. The present invention does not use a pure fluid bed, the sloped nature of the invention leads to the ability to operate with a wide range of material consistencies.